System and method for synchronizing sinusoidal drive to permanent magnet motor without distorting drive voltage

ABSTRACT

A system for controlling a motor ( 3 ) includes a driver circuit ( 5 ) for generating a drive voltage (v) to generate a phase current (i) in the motor. Phase current sensing circuitry ( 10,21 ) digitizes the phase current. A first circuit ( 23 ) provides a reconstructed digital representation of a BEMF signal (v bemf ) of the motor to generate an error-corrected synchronization signal (SYNC) in response to the phase current and a detected error in the motor speed, an amplitude feedback signal ( 15 ), and information (i R ,i L ,Δi L ,v L ) indicative of a resistance (R m ) and an inductance (L m ) of the motor. A motor drive signal ( 15 ) having an error-corrected frequency is generated in response to the synchronization signal. A PWM circuit ( 16 ) produces a PWM signal ( 17 ) having a frequency equal to the error-corrected frequency of the synchronization signal (SYNC) and a duty cycle controlled according to the detected error.

BACKGROUND OF THE INVENTION

The present invention relates generally to reducing the acoustic noisecaused by torque ripple in permanent magnet motors, and moreparticularly to reducing the acoustic noise without “tri-stating” thesinusoidal motor drive voltage that causes the torque ripple.

Permanent magnet motors generate a sinusoidal back electro-motive force(BEMF), and therefore require a sinusoidal drive signal to ensure lowtorque ripple so the acoustic noise generated by the motor will be low.The motor drive signal ordinarily is “tri-stated” for a time interval tosynchronize the BEMF and a drive signal “profile” applied to drive themotor. This is necessary in order to measure the BEMF of the motor. Thetri-stating of the motor drive voltages causes them to abruptly go tozero during the tri-stating interval, and this in turn causes the phasecurrent to also go to zero during the tri-stating interval. The outputsof push-pull driver circuits (not shown) included in a motor drivercircuit 5 (see subsequently described FIG. 2) are tri-stated, by turningoff both the pull-up transistors and pull-down transistors thereof sothat the drive voltage conductors are no longer being driven by themotor driver circuit. Since the output of each push-pull driver istri-stated, it conducts a voltage that represents the motor BEMF. Thesegment A in subsequently described FIG. 1 indicates that the output ofthe push-pull driver is tri-stated, but the voltage generated by themotor on an output conductor of a push-pull driver actually is the motorBEMF.

The abrupt transition of the phase current to zero at the beginning ofthe tri-stating interval causes distortion in the motor phase current.(The term “tri-stating” of the motor drive voltage means that thepull-up transistors and pull-down transistors of the output stages ofthe motor driver circuitry are simultaneously turned off during thetri-stating interval, causing the voltages on the motor drive conductorsto be determined only by the motor during the tri-stating interval.)

The motor torque is equal to the product of the phase current and theBEMF of the permanent magnet motor, so the above mentioned distortion inthe phase current causes a substantial disturbance in the torque ripple,and therefore causes the motor to generate acoustic noise. The acousticnoise is dependent on the motor torque, and if the torque is constant,the acoustic noise is very low. However, if there is substantial rippleor variation in torque it results in a substantial amount of acousticnoise. (The acoustic noise is believed to be generated by smallmechanical deformations caused by the torque ripple in material of whichthe motor is fabricated.)

Prior Art FIG. 1 shows waveforms for the sinusoidal drive voltage, theresulting phase current, and the sensed BEMF of a conventional permanentmagnet motor. As indicated by segment “A” of the motor drive voltagewaveform in FIG. 1, the drive voltage waveform is tri-stated for aninterval during which a segment “B” of both the drive voltage waveformand the phase current waveform are zero. Therefore, the phase current isalso considered to be tri-stated while phase current abruptly goes tozero and remains at zero during the tri-state interval B. After thetri-state interval B, the three waveforms in Prior Art FIG. 1 continuetheir normal sinusoidal variation for the negative portion of thesinusoidal cycle, wherein drive voltage, and hence the phase current,again are tri-stated as they approach the zero crossover level.

Prior Art FIG. 1 also shows the waveform of a sensed signal SYNC whichrepresents the present motor speed. The BEMF voltage continues while asingle drive voltage is being tri-stated by the motor driver circuit.The zero cross-over points of the BEMF voltage typically are sensed bymeans of an ordinary comparator in order to generate the BEMF zerocrossover signal SYNC.

FIG. 1 also shows the torque characteristic of the motor. The normaltorque level is indicated by “C”. The levels “D” of the torquecharacteristic indicate the large torque distortion or ripple caused bythe phase current going to zero during the tri-state intervals B.

Other relevant prior art includes commonly assigned U.S. Pat. No.6,252,362 entitled “Method and Apparatus for Synchronizing PWMSinusoidal Drive to a DC Motor” issued Jun. 26, 2001 to White et al.,incorporated herein by reference. This reference discloses a techniquefor tri-stating and synchronizing drive voltage to a DC permanent magnetmotor to reduce acoustic noise of the motor. When the drive waveformsare properly synchronized, they cause a current in the motor windingsthat is in phase with the BEMF of the motor.

Microcontrollers, such as digital signal processors (DSPs), are commonlyused to control the driver circuits that generate the drive voltagesapplied to permanent magnet motors. Techniques for using digital signalprocessors for sinusoidal driving of permanent magnet DC motors aredisclosed in the technical article “Position Sensorless Brushless DCMotor/Generator Drives: Review and Future Trends” by T. Kim et al., IETElectr. Power Appl., Vol. 1, No. 4, July, 2007. Also see “Sensorless PMBrushless DC Motor Drives” by Nobuyuki Matsui, IEEE Transactions onIndustrial Electronics, Vol. 43, No. 2, April 1996. Also see TexasInstruments Application Report SPRA588 entitled “Implementation of aSpeed Field Oriented Control of 3-Phase PMSM Using TMS320” by ErwanSimon.

Unfortunately, the techniques including use of microcontrollers, DSPs,or the like to generate the drive voltages applied to permanent magnetmotors are very complex, and sometimes include use of the well knownClarke-Park Transformation and PID (proportional-integral-derivative)controllers, and therefore are relatively expensive. The Clarke-ParkTransformation is described in the Texas Instruments Application Report,Literature Number BPRA048, entitled Clarke & Park Transforms on theTMS320C2xx (1997). The Wikipedia article “PID Controller” cited in theInformation Disclosure Statement submitted with the present applicationdescribes PID controller techniques.

Thus, there is an unmet need for a circuit and method for driving apermanent magnet electric motor so as to eliminate acoustic noisewithout tri-stating its drive voltages.

There also is an unmet need for a circuit and method for eliminatingacoustic noise produced by an electric motor without tri-stating a motordrive voltage.

There also is an unmet need for a circuit and method for driving apermanent magnet electric motor without tri-stating the motor drivevoltage wherein parameters of the motor are automatically determined.

There also is an unmet need for a circuit and method for driving apermanent magnet electric motor without the complexity and cost ofutilizing a complex microcontroller such as a DSP.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a circuit and method foreliminating acoustic noise produced by a permanent magnet electric motorwithout tri-stating a motor drive voltage.

It is another object of the invention to provide circuit and method fordriving an electric motor so as to eliminate acoustic noise withouttri-stating its drive voltage.

It is another object of the invention to provide a circuit and methodfor driving a permanent magnet electric motor without tri-stating themotor drive voltage wherein information representative of parameters ofthe motor is automatically determined, or is provided in a non-volatilememory such as an EEPROM.

There also is an unmet need for a circuit and method for driving apermanent magnet electric motor without the complexity and cost ofutilizing a complex microcontroller, such as a DSP.

Briefly described, and in accordance with one embodiment, the presentinvention provides a system for controlling a motor (3), including adriver circuit (5) for generating a drive voltage (v) to generate aphase current (i) in the motor. Phase current sensing circuitry (10,21)digitizes the phase current. A first circuit (23) provides areconstructed digital representation of a BEMF signal (v_(bemf)) of themotor to generate an error-corrected synchronization signal (SYNC) inresponse to the phase current and a detected error in the motor speed,an amplitude feedback signal (15), and information(i_(R),i_(L),Δi_(L),v_(L)) indicative of a resistance (R_(m)) and aninductance (L_(m)) of the motor. A motor drive signal (15) having anerror-corrected frequency is generated in response to thesynchronization signal. A PWM circuit (16) produces a PWM signal (17)having a frequency equal to the error-corrected frequency of thesynchronization signal (SYNC) and a duty cycle controlled according tothe detected error.

In one embodiment, the invention provides a system (1) for controlling apermanent magnet motor (3), including a driver circuit (5) forgenerating a drive voltage (v) applied to the motor (3), the motor (3)generating a phase current (i) in response to the drive voltage (v).Phase current sensing circuitry (10,21) senses the phase current (i) andconverts it to a digitized phase current. A BEMF (back electromotiveforce) reconstruction circuit (23) provides a reconstructed digitalrepresentation of a BEMF signal (v_(bemf)) of the motor (3) to generatean error-corrected synchronization signal (SYNC) in response to thedigitized phase current signal, an amplitude feedback signal (15),information (i_(R),i_(L),Δi_(L),v_(L)) indicative of both a resistance(R_(m)) and an inductance (L_(m)) of the motor (3), and a detected errorin the speed of the motor (3). Signal profile generating circuitry(12,14) generates a motor drive signal (15) having an error-correctedfrequency that is determined by the synchronization signal (SYNC) and isin synchronization with the synchronization signal (SYNC). A PWM (pulsewidth modulation) circuit (16) receiving the motor drive signal (15)produces a PWM signal (17) having a frequency equal to theerror-corrected frequency of the synchronization signal (SYNC) and has aduty cycle controlled in accordance with the detected error in the speedof the motor (3).

In one embodiment, a parameter calculation circuit (22) generates theinformation indicative of both the motor resistance (R_(m)) and themotor inductance (L_(m)) in response to the digitized phase currentunder predetermined conditions. The parameter calculation circuit (22)includes a state machine (19) which performs the functions ofmechanically stopping rotation of a rotor of the motor (03), applyingthe drive voltage (v) as a high frequency sinusoidal signal until thephase current (i) is at a DC value, saving a first value (v_(R)) of theapplied voltage (v), and saving a resulting value (i_(R)) of thedigitized phase current, generating the information indicative of themotor resistance (R_(m)) from the first value (v_(R)) of the appliedvoltage (v) and the resulting value (i_(R)) of the digitized phasecurrent signal, causing the phase current (i) to be equal to zero,saving a measured value of current change (Δi_(L)) and a second value(v_(L)) of the applied drive voltage (v), and generating the informationindicative of the motor inductance (L_(m)) from the measured value ofcurrent change (Δi_(L)) and the second value (v_(L)) of the applieddrive voltage (v). The motor resistance (R_(m)) may be represented bythe expression R_(m)=v_(R)/i_(R) and the motor inductance (L_(m)) may berepresented by the expression

${L_{m} = \frac{v_{L}}{\Delta\;{i_{L}/\Delta}\; t}},$where v_(R) is the first value of the applied voltage (v), i_(R) is theresulting value of the digitized phase current, v_(L) is the secondvalue of the applied drive voltage (v), Δi_(L) is the measured value ofcurrent change, and Δt is a time interval.

In one embodiment, the BEMF reconstruction circuit (23) includes a statemachine (20) which operates to capture a digitized phase current (I), achange (ΔI) in the digitized phase current (I), and a digitized motordriving voltage (V). The BEMF construction circuit (23) operates tocompute a value of the BEMF signal (v_(bemf)) in accordance with theexpressionk·v _(bemf) =V·i _(R) ·Δi _(L) −I·v _(R) −v _(L) ·ΔI,where V is the digitized motor driving voltage, I is the digitized phasecurrent, ΔI is a change in the digitized phase current, i_(R) is acurrent indicative of the resistance (R_(m)) of the motor, v_(R) is avoltage indicative of the resistance (R_(m)), Δi_(L) is a change of acurrent indicative of the inductance (L_(m)) of the motor, and v_(L) isa voltage indicative of the inductance (L_(m)) of the motor.

In a described embodiment, the state machine (20) operates to cause thesynchronization signal (SYNC) to be at a first logic level (“0”) if aMSB (most significant bit) of the BEMF signal (v_(bemf)) is a “0” and tocause the synchronization signal (SYNC) to be at a second logic level(“1”) if the MSB of the BEMF signal (v_(bemf)) is a “1”. In a describedembodiment, another state machine (19) generates the information(i_(R),i_(L),Δi_(L),v_(L)) indicative of the resistance (R_(m)) and theinductance (L_(m)) of the motor (3) in response to the phase current.

In a described embodiment, the signal profile generating circuitry(12,14) includes an electrical period measuring circuit (12) formeasuring an amount of time between corresponding edges of successivepulses of the synchronization signal (SYNC).

In one embodiment, the motor (3) is a three phase motor, and the phasecurrent (i) is generated in response to a single drive voltage (v). Inone embodiment, the signal profile generating circuit (12,14) generatesthe motor drive signal (15) as a sinusoidal signal. In anotherembodiment, the signal profile generating circuit (12,14) generates themotor drive signal (15) as a trapezoidal signal.

In one embodiment, the invention provides a method for controlling apermanent magnet motor (3), including generating a drive voltage (v)applied to the motor (3), the motor (3) generating a phase current (i)in response to the drive voltage (v); sensing the phase current (i) andconverting it to a digitized phase current signal (I); providinginformation indicative of both a motor resistance (R_(m)) and a motorinductance (L_(m)); generating a reconstructed digital representation ofa BEMF signal (v_(bemf)) produced in the motor (3) in response to anamplitude feedback signal (15), the digitized phase current signal (I),the information indicative of the motor resistance (R_(m)) and the motorinductance (L_(m)), and a detected difference between a speed of themotor (3) and a desired speed of the motor (3) in order to produce asynchronization signal (SYNC) having an error-corrected frequency thatis determined by zero crossover points of the reconstructed digitalrepresentation of the BEMF signal (v_(bemf)); using a drive signalprofile to generate a digital motor drive signal (15) having theerror-corrected frequency; and producing a PWM signal (17) having afrequency equal to the error-corrected frequency and also having a dutycycle controlled according to the detected difference between the speedof the motor (3) and the desired speed of the motor (3).

In one embodiment, the method includes operating the BEMF reconstructioncircuit (23) to compute a value of the BEMF signal (v_(bemf)) inaccordance with the expressionk·v _(bemf) =V·i _(R) ·Δi _(L) −I·v _(R) −v _(L) ·ΔI,where V is the digitized motor driving voltage, I is the digitized phasecurrent, ΔI is a change in the digitized phase current, i_(R) is acurrent indicative of the resistance (R_(m)) of the motor, v_(R) is avoltage indicative of the resistance (R_(m)), Δi_(L) is a change of acurrent indicative of the inductance (L_(m)) of the motor, and v_(L) isa voltage indicative of the inductance (L_(m)) of the motor. In oneembodiment, the BEMF reconstruction circuit (23) includes a statemachine (20) which operates to capture the digitized phase current (I),the change in the digitized phase current (ΔI), and the digitized motordriving voltage (V), the method including operating the state machine(20) to cause the synchronization signal (SYNC) to be at a first logiclevel (“0”) if a MSB (most significant bit) of the BEMF signal(v_(bemf)) is a “0” and to cause the synchronization signal (SYNC) to beat a second logic level (“1”) if the MSB of the BEMF signal (v_(bemf))is a “1”.

In one embodiment, the method includes operating the state machine (20)to cause the synchronization signal (SYNC) to be at a first logic level(“0”) if a MSB (most significant bit) of the BEMF signal (v_(bemf)) is a“0” and to cause the synchronization signal (SYNC) to be at a secondlogic level (“1”) if the MSB of the BEMF signal (v_(bemf)) is a “1”.

In one embodiment, the method includes operating the signal profilegenerating circuit (12,14) to generate the motor drive signal (15) as asinusoidal signal.

In one embodiment, the invention provides a system (1) for controlling apermanent magnet motor (3), including means (5) for generating a drivevoltage (v) applied to the motor (3), the motor (3) generating a phasecurrent (i) in response to the drive voltage (v); means (10,21) forsensing the phase current (i) and converting it to a digitized phasecurrent signal (I); means (22) for providing information indicative ofboth a motor resistance (R_(m)) and a motor inductance (L_(m)); means(23) for generating a reconstructed digital representation of a BEMFsignal (v_(bemf)) produced in the motor (3) in response to an amplitudefeedback signal (15), the digitized phase current signal (I), theinformation indicative of the motor resistance (R_(m)) and the motorinductance (L_(m)), and a detected difference between a speed of themotor (3) and a desired speed of the motor (3) in order to produce asynchronization signal (SYNC) having an error-corrected frequency thatis determined by zero crossover points of the reconstructed digitalrepresentation of the BEMF signal (v_(bemf)); means (12,14) for using adrive signal profile to generate a digital motor drive signal (15)having the error-corrected frequency; and means (16) for producing a PWMsignal (17) having a frequency equal to the error-corrected frequencyand also having a duty cycle controlled according to the detecteddifference between the speed of the motor (3) and the desired speed ofthe motor (3).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a timing diagram useful in explaining the shortcomings of theclosest prior art.

FIG. 2 is a block diagram of a circuit for driving a permanent magnetmotor in accordance with the present invention.

FIG. 3A is diagram of a state machine included in block 22 of FIG. 2.

FIG. 3B is diagram of a state machine included in block 23 of FIG. 2.

FIG. 4 is a flow diagram useful in describing the method associated withthe circuit described with reference to FIG. 2 and the state machinesshown in FIGS. 3A and 3B.

FIG. 5 is a timing diagram useful in describing the operation of thecircuit of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a way of eliminating the above-mentionedproblem of acoustic noise caused by phase current distortion inpermanent magnet motors driven by the closest prior art motorcontrollers. This is accomplished by a technique of “reconstructing” theBEMF signal used to synchronize the driving of a permanent magnet motorwithout tri-stating its drive voltage. Consequently, there is noassociated distortion of the motor phase current, and therefore there isno corresponding distortion of the motor torque which causes theacoustic noise.

Referring to FIG. 2, system 1 includes a permanent magnet motor 3 drivenby a motor driver circuit 5. A motor controller circuit 7 receives asensed phase current “i” of motor 3 through conductor 11 and produces aPWM (pulse width modulated) motor control signal on conductor 17 as aninput to driver circuit 5. Sensed phase current i may be generated inconductor 11 by a conventional current sensor 10 that is either in orconnected to motor 3. Control system 7 senses the phase current i anduses it to provide feedback from motor 3 to motor driver circuit 5. Thefeedback includes motor speed/frequency and phase information in theform of a synchronization signal SYNC on conductor 24 and amplitudeinformation on bus 15.

Driver circuit 5 generates three-phase sinusoidal drive voltages onconductors 9-1, 9-2, and 9-3, respectively, in response to theabove-mentioned feedback. (However, note that the invention is just asapplicable to a single-phase permanent magnet motor as to a three-phasepermanent magnet motor.) The driver circuits (not shown) in block 5 areconventional.

The circuitry in motor controller 7 that generates the above mentionedfeedback includes a phase current sensing circuit 21 which receives thesingle phase current i generated in conductor 11 by phase current sensor10 in response to current in motor 3. Motor controller 7 receives phasecurrent i and provides PWM (pulse width modulation) feedback onconductor 17 to motor driver circuit 5. Motor controller 7 determinesthe amount of an error in the timing of SYNC, determined on the basis ofthe present phase current i and the present output 15 of sine drivingprofile circuit 14. Motor 3 is driven to its maximum speed, limited bythe power supply. Since motor 3 does not have a terminal from which thephase current i can be directly conducted, current sensor 10, which canbe either internal or external to motor 3, is used to determine thephase current i. Phase current sensing circuit 21 performs ananalog-to-digital conversion of the analog phase current i to a digitalrepresentation which indicates values of i as a function of time.(Therefore, it can be seen that the phase current sensing according tothe invention is very different than the phase current sensing in priorart permanent magnet motor controllers with waveforms such as thoseshown in Prior Art FIG. 1.)

The digital information generated by phase current sensing circuit 21 isreceived and utilized by a parameter calculation circuit 22 and a BEMFreconstruction circuit 23 to perform the function of “reconstructing”the BEMF signal, for example by means of suitable state machines. Inblock 22 of FIG. 2, information representative of the motor resistanceR_(m) and motor inductance L_(m) is determined, for example, by means ofa state machine such as the one shown in subsequently described FIG. 3A.The foregoing information representative of R_(m) and L_(m) of motor 3is computed automatically, as indicated in blocks 30-40 of the flowchartof subsequently described FIG. 4. (Alternatively, the informationrepresentative of R_(m) and L_(m) may be previously determined andstored in block 22 in place of a parameter calculation circuit.)

Then, using (in effect) the foregoing values of R_(m) and L_(m) alongwith feedback generated on bus 15 by sine driving profile circuit 14,BEMF reconstruction circuit 23 in FIG. 2 digitally reconstructs the BEMFsignal, determines its zero-crossing times, and generates rising edgesof the synchronization signal SYNC on conductor 24 in response to thezero-crossing times. The frequency of SYNC therefore represents theinstantaneous actual frequency (rotor speed) of motor 3. Any error inthe motor speed may be determined by comparing it with the previouslymeasured value representing the desired motor speed.

The BEMF zero-crossing signal SYNC on conductor 24 is applied to aninput of an electrical period measurement circuit 12, the output 13 ofwhich is connected to the input of a sine drive profile circuit 14. Thefrequency information contained in SYNC is converted to an electricalperiod measurement by circuit 12, and is presented on digital bus 13 asan input to sine driving profile circuit 14. The digital information onmulti-conductor digital bus 13 represents error in the motor speed,which is corrected in response to the feedback provided by SYNC, becauseSYNC represents the variation in the frequency of motor 3. Sine drivingprofile circuit 14 stores a normalized sine wave and is able tointerpret the electrical period measurement from circuit 12 so that sinedriving profile circuit 14 produces a sine wave profile having thecorrect frequency.

Sine driving profile circuit 14 generates digital data that represents asine wave on multi-conductor digital bus 15. The frequency of thedigital sine wave data on digital bus 15 is adjusted in response toshifts in the frequency of SYNC. The measured present speed is in effectcompared to the previously measured desired speed. If the motor iseither accelerating or decelerating, there will be error between thepreviously measured desired speed and the currently measured speed, andfeedback causes automatic correction of the error. The shifts in thefrequency of SYNC are caused by changes in the values of phase current iand are also caused by an error or difference between a previous valueof the sine driving profile information on bus 15 and a previouslymeasured value representing the desired speed of motor 3. The period ofSYNC is adjusted to minimize this error or difference in order toaccomplish the correction. The sinusoidal driving profile amplitudecontrol information on bus 15 in effect provides feedback that causeselectrical period measurement circuit 12 to adjust the “ideal”sinusoidal profile information stored in sine driving profile circuit 14to cause it to generate a corrected output on multi-conductor digitalbus 15 of sine driving profile circuit 14.

In block 23 of FIG. 2, the BEMF signal of motor 3 is reconstructedwithin controller 7 in accordance with subsequently described Equations1-6, which provide the basis for generating the data required toreconstruct the digital BEMF data signal that represents the “actual”analog BEMF signal of motor 3. The times at which the reconstructed BEMFsignal crosses the zero volt level can be readily detected from thereconstructed BEMF data. The detected BEMF zero-crossing times then areutilized to adjust the timing between the SYNC pulses generated onconductor 24, which are coupled to an input of electrical periodmeasurement circuitry 12 in FIG. 2. The waveform of SYNC is shown insubsequently described FIG. 5.

Electrical period measurement circuitry 12 then utilizes a counter (notshown) to measure the amount of time between corresponding edges of thepulses of synchronization signal SYNC, i.e., to measure the period ofSYNC. Electrical period measurement circuit 12 then generates a feedbacksignal on digital bus 13 which represents the present speed of motor 3.

The signal on bus 13 represents the corrected speed of motor 3 and issynchronized with SYNC and the rotation of motor 3, and is utilized bysine driving profile circuit 14 to adjust the instantaneous amplitudesof the stored ideal sinusoidal driving profile information so as togenerate the sinusoidal digital motor drive voltage profile on digitalbus 15 as needed to maintain the desired motor speed.

The information on bus 15 includes digital sinusoidal signal informationincluding both waveform amplitude information and synchronizationinformation generated in response to the signal SYNC produced by BEMFreconstruction circuit 23 to control the frequency, phase, and amplitudeof each of the three motor driver voltages. Motor 3 is driven by a puresine wave signal because the digital amplitude information generated bysine driving profile circuit 14 represents a pure analog sine wavesignal that rises from zero to a maximum positive voltage and then fallsback through zero to a minimum negative voltage before returning tozero.

The digital signal on bus 15 is utilized by a conventional PWMgeneration circuit 16 to determine the duty cycle of a PWM signal 17generated by PWM generation circuit 16. PWM circuit 16 generates pulsesthe widths of which have duty cycles as needed to adjust the amplitudesof the motor drive voltage signals. PWM generation circuit 16 may use astandard method of converting a signal amplitude to a duty cycle,including comparing the amplitude value with a triangular waveform. Whenthe value of the digital signal on bus 15 is less than the value of thetriangular waveform, the PWM signal generated on conductor 17 is low,and when the value of the triangular waveform is higher than the valueof the digital signal on bus 15, the PWM signal generated on conductor17 is high. The frequency of the provided triangular waveform is fixed.Therefore, as the amplitude of the digital signal on bus 15 increases,the duty cycle of the PWM signal generated on conductor 17 increases,and vice versa.

PWM signal 17 is utilized in motor driver circuit 5 to control theturning on and turning off of pull-up output transistors and pull-downtransistors in the output stages of the three push-pull driver circuits(not shown) included in motor driver circuit 5. The pull-up andpull-down output transistors in motor driver circuit 5 are turned on andoff in accordance with the duty cycle of PWM signal 17 and therebycontrol the amplitude, frequency, and phase of each of the threesinusoidal motor drive voltages on conductors 9-1, 9-2, and 9-3,respectively.

Before describing FIGS. 3A, 3B, and 5, it may be helpful to present theanalytical basis for the motor parameter calculations performed byparameter calculation circuit 32 and the BEMF reconstructioncalculations performed by BEMF reconstruction circuit 23.

The motor speed (i.e., angular velocity of the rotor of motor 3) is usedto achieve the driving of motor 3 in synchronization with its BEMF. Thephase current i of motor 3 does not directly provide the motor speed.However, information representative of the values of L_(m) and R_(m) maybe derived from the digitized representation of phase current i. Thedigitized phase current i is used to reconstruct the BEMF of motor 3.The reconstructed BEMF signal then represents the actual BEMF signalgenerated by motor 3 on one of the tri-stated drive voltage conductors9-1,2,3, and hence represents the speed of motor 3. The reconstructedBEMF signal also synchronizes the motor drive voltages generated bymotor driver 5 with the BEMF of motor 3.

Theoretically, the speed of motor 3 may be derived from the measuredphase current data by means of the following “motor equation”:

$\begin{matrix}{{{\overset{\_}{v}}_{bemf} = {\overset{\_}{v} - {\overset{\_}{i}\; R_{m}} - {L_{m}\frac{\mathbb{d}\overset{\_}{i}}{\mathbb{d}t}}}},} & \left( {{Eqn}.\mspace{14mu} 1} \right)\end{matrix}$in which v _(bemf) is the BEMF of the driven motor 3 and is proportionalto its speed. v is a phase output voltage vector of the driven motor 3,and ī is the measured phase current vector of the driven motor. R_(m) isthe stator resistance of the driven motor, and L_(m) is the inductanceof the stator winding of the driven motor 3.

Equation 1 depends on not only the driven value v and the measured valueī, but also on the constant motor parameters R_(m) and L_(m) of motor 3.(Note that since these motor parameters vary from motor to motor, theirvalues may be determined for each motor.)

For the case in which the phase current is sampled at a fixed frequencyor motor speed, Equation 1 may be written as

$\begin{matrix}{v_{bemf} = {v - {iR}_{m} - {L_{m}{\frac{\Delta\; i}{\Delta\; t}.}}}} & \left( {{Eqn}.\mspace{14mu} 2} \right)\end{matrix}$In the case wherein motor 3 is stationary (see block 30 of subsequentlydescribed FIG. 4), v_(bemf) is equal to zero, so Equation 2 is reducedtov _(bemf)=0=v _(R) −i _(R) ×R _(m),  (Eqn. 3)where v_(R) and i_(R) are the values of v and i utilized to computeR_(m). Consequently, R=v_(R)/i_(R).

The motor inductance L_(m) is obtained from the measured current changeΔi_(L) and the value of the corresponding driven voltage v_(L) whilei=0, where v_(L) and i_(L) are the values of v and i utilized to computeL_(m). In this case Equation 2 is reduced to

$\begin{matrix}{{0 = {v_{L} - {L_{m}{\frac{\Delta\; i_{L}}{\Delta\; t}.{Consequently}}}}},{L_{m} = \frac{v_{L}}{\Delta\;{i_{L}/\Delta}\; t}},} & \left( {{Eqn}.\mspace{14mu} 4} \right)\end{matrix}$where Δi_(L) is the difference in the measured phase current during asuitable time internal Δt. Then, as indicated in block 40, the measuredcurrent change Δi_(L) and the applied voltage v_(L) are saved. Δt is thePWM period, and is constant during the parameter calculation and drivingof motor 3. And as indicated by Equation 5 below, Δt is cancelled out.

Blocks 41-47 of subsequently described FIG. 4 indicate how theinformation indicative of the values of R_(m) and L_(m) is used by thestate machine shown in subsequently described FIG. 3B in order to obtainthe above-mentioned reconstructed BEMF data.

To accomplish this, Equations (3) and (4) are substituted into Equation(2), which results in

$\begin{matrix}{v_{bemf} = {v - {i\frac{v_{R}}{i_{R}}} - {\frac{v_{L}}{\Delta\;{i_{L}/\Delta}\; t} \times {\frac{\Delta\; i}{\Delta\; t}.}}}} & {{Eqn}.\mspace{14mu} 5}\end{matrix}$Multiplying Equation 5 by i_(R) results ink·v _(bemf) =v·i _(R) ·Δi _(L) −i·v _(R) −v _(L) ·Δi,  Eqn. 6where k×v_(bemf) is the motor speed, k is equal to i_(R)×Δi_(L). (Thevalue of k is not important since it is just a gain value, whereas theinformation of interest is the period information of v_(bemf).)

From Equation 6, the motor speed k×v_(bemf) can be determined withoutthe complex calculations which are required by the closest prior artmotor control systems, and without having to tri-state the driver outputwhich are also required by the closest prior art motor control systems.

In Equation 6, the instantaneous amplitude of motor drive voltage v isadjusted as needed to provide the required amount of drive to motor 3 tomaintain its desired speed despite any variations in the load driven bymotor 3 and despite any variations in certain other parameters. Digitalvalues corresponding to the analog motor drive voltage v on conductors9-1,2,3 are generated on bus 15 and are in accordance with Equation 6.

FIG. 3A shows a diagram of a state machine which can be included inblock 22 of FIG. 2 to obtain the values of v, i, Δi, and Δt needed todetermine the values of the constant motor parameters R_(m) and L_(m).Referring to FIG. 3A, a state machine 19 for obtaining the informationrequired to determine R_(m) and L_(m) includes states 25, 26, 27 and 28.State 25 is an initial “brake” or “standby” state that occurs when acondition “reset” is met, i.e., when “reset” goes from a “1” to a “0”.State machine 19 then goes from state 25 to state 26. In state 26, whena condition “deltaIisZero” is met, i.e., goes from a “0” to a “1”, statemachine 19 causes the values v_(R) and i_(R) to be obtained, and thenstate machine 19 goes from state 26 to state 27. In state 27, when acondition “IisZero” goes from a “0” to a “1”, state machine 19 obtainsthe present values of Δi_(L) and v_(L). State machine 19 then proceedsfrom state 27 to state 28. In state 28, state machine 19 causes motor 3to run until a condition “Fault=0”, which means that there is no errorand therefore motor 3 is running perfectly, changes to “Fault=1”. Thecondition “Fault=1” may mean that motor 3 is not running, or there isexcessively high motor current, or an excessively low drive voltage, orsome other unacceptable fault condition exists. Then state machine 19returns to state 25.

FIG. 3B shows a diagram of a state machine 20 which can be included inBEMF reconstruction circuit 23 in FIG. 2 to generate the feedback andsynchronization signal SYNC. State machine 20 includes states 50, 51,52, 53, 54, and 55. “Start” state 50 is set when a condition “start”goes from a “0” to a “1”, and this causes state machine 20 to go tostate 51. In state 51, state machine 20 captures the digitized value ofa present value of the motor phase current i when a condition“capCurrReady” goes from a “0” to a “1”. (Note that phase current ialready has been digitized by phase current sensing circuit 21 in FIG.2.) State machine 20 then goes to state 52 and captures the digitizedpresent value of motor drive voltage v when the condition “capVoltReady”goes from a “0” to a “1”. State machine 20 then goes to state 53 andcalculates the BEMF signal v_(bemf) according to Equation 6, and thengoes to state 54 if MSB is equal to “0” and sets SYNC to a “0” asindicated by state 54. If MSB is a “1”, state machine 20 sets SYNC to a“1”. (MSB is the most significant bit of the computed digitized signalv_(bemf)). In either case, state machine 20 goes to state 51 and againcaptures a new digitized present value of phase current i when“capCurrReady” goes from a “0” to a “1”. What this means is thatv_(bemf) is compared with a provided reference value that represents thedesired speed of motor 3. If v_(bemf) is greater than the desired motorspeed reference value, then SYNC is set to a “1”, and if v_(bemf) isless than the desired motor speed reference value, then SYNC is set to a“0”.

Blocks 30-40 of FIG. 4 constitute a flowchart of the basic processperformed by state machine 19 of FIG. 3A in parameter calculationcircuit 22 of FIG. 2 to determine information representative of theconstant parameters R_(m) and L_(m) of motor 3. In block 30 of FIG. 4,the running motor 3 is forced into a stationary or standby mode 3 byapplying a mechanical brake to stop the rotor of motor 3 to ensure thatthe BEMF signal v_(bemf) of Equation 2 is zero. In order to measureR_(m) and L_(m), a high frequency sinusoidal drive voltage is applied tomotor 3, as indicated in block 32. Since the rotor of motor 3 is forcedto be stationary, the applied high frequency drive voltage v does notmove the rotor of motor 3, thereby ensuring that v_(bemf) remains equalto zero. The loop associated with decision block 34 of FIG. 4 then waitsuntil the computed value of Δi is equal to zero. Then, as indicated inblock 36, the value of the digitized “measured phase current” i_(R) andthe value of the digitized “applied drive voltage” v_(R) needed tocompute R_(m) are saved. The motor resistance R_(m) may be determinedfrom the measured current i_(R) and the “driven voltage” v_(R) when thecondition i=0 in block 38 is met.

State machine 19 of FIG. 3A remains in state 26 as long as the conditiondeltaIisZero=“0” continues. This is when the steps of blocks 36 and 38in the flowchart of FIG. 4 have been completed. An idle loop associatedwith decision block 38 waits for the computed value of phase current ito be equal to zero. Then the value of the “measured current change”Δi_(L) and the current i_(L) needed to compute L_(m) are determined andsaved, as indicated in block 40. When state 27 of state machine 19 iscomplete, that means the step in block 40 of FIG. 4 has been completed.

Blocks 41-47 of subsequently described FIG. 4 indicate how the foregoinginformation representative of the values of R_(m) and L_(m) is used bystate machine 20 in FIG. 3B of BEMF reconstruction circuit 23 of FIG. 2,to provide the reconstructed BEMF data. In block 41 of FIG. 4, therunning of motor 3 is started by the state machine of FIG. 3B. Indecision block 42, the state machine determines whether a variableSAMPLE DATA is equal to “1”. The data is sampled at a fixed frequency.Therefore, at the end of every fixed period Δt, SAMPLE DATA goes to “1”to sample the data. If the determination of decision block 42 isaffirmative, state machine 20 goes to block 43 and captures thedigitized present motor current and the rate of change (during aninterval Δt) of the present motor current to obtain the digitized valuesI and ΔI. If the determination of decision block 42 is negative, statemachine 20 waits until SAMPLE DATA is equal to a “1”. In block 44, statemachine 20 captures the motor drive voltage to obtain the digitizedvalue V. Next, state machine 20 goes to block 45 and computes thedigital value of v_(bemf) in accordance with Equation 6. Then, statemachine 20 goes to decision block 46 and determines if the MSB (mostsignificant bit) of the digital value of v_(bemf) is greater than orequal to 0, to cause SYNC to be generated. If the determination ofdecision block 46 is affirmative, the state machine goes to block 47 andcaptures the digital value of v_(bemf) and returns to the entry point ofblock 41. If the determination of decision block 46 is negative, theprogram returns to the entry point of block 41.

The previously measured motor speed represented by the frequency of SYNCin FIG. 2 is used as the basis for the current voltage profilegeneration. If motor 3 is accelerating or decelerating (e.g., due to thevarying load), the frequency of motor drive voltage profile generated bysine driving profile circuit 14 automatically changes to adapt to thespeed change of the accelerating or decelerating motor and causes PWMgeneration circuit 15 to modify the amplitudes of the motor drivevoltages accordingly. Therefore, there is no need for the previouslymentioned complex PID control calculations which are required in theclosest prior art motor controllers.

Thus, the BEMF of motor 3 as a function of time is calculated on thebasis of Equations 1-6 and used to determine the present BEMFzero-crossing times and the present motor speeds without having totri-state the motor drive voltages. In the above described example,motor 3 is driven with a pure sinusoidal pattern having a frequencyequal to the frequency of the measured speed of motor 3 adjusted so asto be equal to the frequency of the desired motor speed in response tothe signal SYNC shown in FIG. 5. This sinusoidal pattern is updatedevery AC cycle, and the zero-crossing points at which the calculatedBEMF goes from negative to positive and vice versa are used forsynchronization of the motor drive signal with the reconstructed BEMFsignal.

FIG. 5 shows waveforms for the sinusoidal drive voltage v, the resultingphase current i, and the voltage represented by the reconstructed BEMFof permanent magnet motor 3 in FIG. 2. Note that segments “A” and “B”shown in Prior Art FIG. 1 are missing from the waveforms shown in FIG. 5because no tri-stating of the motor drive voltages occurs in permanentmagnet motor 3 in FIG. 2. Therefore, there is no correspondingdistortion of the phase current waveform shown in FIG. 5. As previouslymentioned, the torque of a permanent magnet motor is equal to theproduct of its BEMF (back electromotive force) and its phase current. Asillustrated by the constant, ripple-free torque curve shown in FIG. 5,there is none of the large torque variation due to the phase currentdistortion seen in Prior Art FIG. 1. Consequently, essentially noacoustic noise is generated by motor 3.

To summarize, the phase current and the motor parameters in theforegoing example of the invention are provided as inputs to a statemachine (or other suitable circuitry) to reconstruct the BEMF signal ona real-time basis, and to extract the zero-crossing information from thereconstructed BEMF signal to generate the synchronization signal SYNC.The drive voltages of the motor 3 are synchronized with thereconstructed BEMF data without tri-stating the motor drive voltages onconductors 9-1, 9-2, and 9-3, respectively. Distortion in the torqueproduced by motor 3 by the prior art technique of sensing the actualBEMF zero-crossings to achieve synchronization of the drive voltage andthe BEMF of the motor phase current is essentially eliminated.Therefore, and in contrast to the closest prior art, there is nocorresponding distortion in the phase current and consequently noacoustic noise.

Since the described control scheme makes use of the motor speedrepresented by the SYNC signal, the disclosed motor control system 1 ofFIG. 2 eliminates the need for the previously mentioned complexcalculations that are required in the closest prior art DC permanentmagnet motor control systems. Furthermore, the informationrepresentative of the motor parameters L_(m) and R_(m) is automaticallydetermined. Furthermore, the above described technique works for bothsinusoidal and trapezoidal BEMF motors.

Although a sinusoidal BEMF motor has been described, the method andequations are not limited to a sinusoidal BEMF motor. The samescheme/method may be used for a trapezoidal BEMF motor (although theimprovement in acoustic performance may not be as great as for asinusoidal BEMF motor). This is in contrast to methods that use a DSP(digital signal processor) and the previously mentioned Clarke-Parktransformation, wherein the motors BEMF must be sinusoidal.

While the invention has been described with reference to severalparticular embodiments thereof, those skilled in the art will be able tomake various modifications to the described embodiments of the inventionwithout departing from its true spirit and scope. It is intended thatall elements or steps which are insubstantially different from thoserecited in the claims but perform substantially the same functions,respectively, in substantially the same way to achieve the same resultas what is claimed are within the scope of the invention.

It should be understood that the invention may be used to reduceacoustic noise in various motor applications, including cooling fans forconsumer electronic devices, ceiling fans, cooling fans for homeappliances, and the like. The invention is also applicable to brushlesspermanent magnet DC motors (i.e., motors having a trapezoidal BEMF).

What is claimed is:
 1. A system for controlling a permanent magnetmotor, comprising: (a) a driver circuit for generating a drive voltageapplied to the motor, the motor generating a phase current in responseto the drive voltage; (b) phase current sensing circuitry for sensingthe phase current and converting it to a digitized phase current; (c) aBEMF (back electromotive force) reconstruction circuit for providing areconstructed digital representation of a BEMF signal of the motor togenerate an error-corrected synchronization signal in response to thedigitized phase current signal, an amplitude feedback signal,information indicative of both a resistance and an inductance of themotor, and a detected error in a speed of the motor; (d) signal profilegenerating circuitry for generating a motor drive signal having anerror-corrected frequency that is determined by the synchronizationsignal and is in synchronization with the synchronization signal; and(e) a PWM (pulse width modulation) circuit receiving the motor drivesignal for producing a PWM signal having a frequency equal to theerror-corrected frequency of the synchronization signal and having aduty cycle controlled in accordance with the detected error in the speedof the motor, a parameter calculation circuit for generating theinformation indicative of both the motor resistance and the motorinductance in response to the digitized phase current underpredetermined conditions, wherein the parameter calculation circuitincludes a state machine which performs the functions of mechanicallystopping rotation of a rotor of the motor, applying the drive voltage asa high frequency sinusoidal signal until the phase current is at a DCvalue, saving a first value of the applied voltage, and saving aresulting value of the digitized phase current, generating theinformation indicative of the motor resistance from the first value ofthe applied voltage and the resulting value of the digitized phasecurrent signal, causing the phase current to be equal to zero, saving ameasured value of current change and a second value of the applied drivevoltage, and generating the information indicative of the motorinductance from the measured value of current change and the secondvalue of the applied drive voltage.
 2. The system of claim 1 wherein themotor resistance is represented by the expression R_(m)=v_(R)/i_(R) andwherein the motor inductance is represented by the expression${L_{m} = \frac{v_{L}}{\Delta\;{i_{L}/\Delta}\; t}},$ where v_(R) is thefirst value of the applied voltage, i_(R) is the resulting value of thedigitized phase current, v_(L) is the second value of the applied drivevoltage, Δi_(L) is the measured value of current change, and Δt is atime interval.
 3. The system of claim 1 wherein the BEMF reconstructioncircuit includes a state machine which operates to capture a digitizedphase current, a change in the digitized phase current, and a digitizedmotor driving voltage.
 4. The system of claim 3 wherein the BEMFconstruction circuit operates to compute a value of the BEMF signal inaccordance with the expressionk·v _(bemf) =V·i _(R) ·Δi _(L) −I·v _(R) −v _(L) ·ΔI, where V is thedigitized motor driving voltage, I is the digitized phase current, ΔI isa change in the digitized phase current, i_(R) is a current indicativeof the resistance of the motor, v_(R) is a voltage indicative of theresistance, Δi_(L) is a change of a current indicative of the inductanceof the motor, and v_(L) is a voltage indicative of the inductance of themotor, and wherein k is i_(R)×Δi_(L).
 5. The system of claim 4 whereinthe state machine operates to cause the synchronization signal to be ata first logic level if a MSB (most significant bit) of the BEMF signalis a “0” and to cause the synchronization signal to be at a second logiclevel if the MSB of the BEMF signal is a “1”.
 6. The system of claim 5including another state machine for generating the informationindicative of the resistance and the inductance of the motor in responseto the phase current.
 7. The system of claim 1 wherein the signalprofile generating circuitry includes an electrical period measuringcircuit for measuring an amount of time between corresponding edges ofsuccessive pulses of the synchronization signal.
 8. The system of claim1 wherein the phase current sensing circuit includes an external currentsensor for sensing the phase current.
 9. The system of claim 1 whereinthe motor is a three phase motor, and the phase current is generated inresponse to a single drive voltage.
 10. The system of claim 1 whereinthe signal profile generating circuit generates the motor drive signalas a sinusoidal signal.
 11. The system of claim 1 wherein the signalprofile generating circuit generates the motor drive signal as atrapezoidal signal.
 12. A method for controlling a permanent magnetmotor, comprising: (a) generating a drive voltage applied to the motor,the motor generating a phase current in response to the drive voltage;(b) sensing the phase current and converting it to a digitized phasecurrent signal; (c) providing information indicative of both a motorresistance and a motor inductance; (d) generating a reconstructeddigital representation of a BEMF signal produced in the motor inresponse to an amplitude feedback signal, the digitized phase currentsignal, the information indicative of the motor resistance and the motorinductance, and a detected difference between a speed of the motor and adesired speed of the motor in order to produce a synchronization signalhaving an error-corrected frequency that is determined by zero crossoverpoints of the reconstructed digital representation of the BEMF signal;(e) using a drive signal profile to generate a digital motor drivesignal having the error-corrected frequency; and (f) producing a PWMsignal having a frequency equal to the error-corrected frequency andalso having a duty cycle controlled according to the detected differencebetween the speed of the motor and the desired speed of the motor, andoperating the BEMF reconstruction circuit to compute a value of the BEMFsignal in accordance with the expression:k·v _(bemf) =V·i _(R) ·Δi _(L) −I·v _(R) −v _(L) ·ΔI, where V is thedigitized motor driving voltage, I is the digitized phase current, ΔI isa change in the digitized phase current, i_(R) is a current indicativeof the resistance of the motor, v_(R) is a voltage indicative of theresistance, Δi_(L) is a change of a current indicative of the inductanceof the motor, and v_(L) is a voltage indicative of the inductance of themotor, and k is equal to i_(R)×Δi_(L).
 13. The method of claim 12including operating the state machine to cause the synchronizationsignal to be at a first logic level if a MSB (most significant bit) ofthe BEMF signal is a “0” and to cause the synchronization signal to beat a second logic level if the MSB of the BEMF signal is a “1”.
 14. Themethod of claim 12 including operating the signal profile generatingcircuit to generate the motor drive signal as a sinusoidal signal. 15.The method of claim 12 wherein the BEMF reconstruction circuit includesa state machine which operates to capture the digitized phase current,the change in the digitized phase current, and the digitized motordriving voltage, the method including operating the state machine tocause the synchronization signal to be at a first logic level if a MSB(most significant bit) of the BEMF signal is a “0” and to cause thesynchronization signal to be at a second logic level if the MSB of theBEMF signal is a “1”.
 16. A system for controlling a permanent magnetmotor, comprising: (a) means for generating a drive voltage applied tothe motor, the motor generating a phase current in response to the drivevoltage; (b) means for sensing the phase current and converting it to adigitized phase current signal; (c) means for providing informationindicative of both a motor resistance and a motor inductance; (d) meansfor generating a reconstructed digital representation of a BEMF signalproduced in the motor in response to an amplitude feedback signal, thedigitized phase current signal, the information indicative of the motorresistance and the motor inductance, and a detected difference between aspeed of the motor and a desired speed of the motor in order to producea synchronization signal having an error-corrected frequency that isdetermined by zero cross-over points of the reconstructed digitalrepresentation of the BEMF signal; (e) means for using a drive signalprofile to generate a digital motor drive signal having theerror-corrected frequency; and (f) means for producing a PWM signalhaving a frequency equal to the error-corrected frequency and alsohaving a duty cycle controlled according to the detected differencebetween the speed of the motor and the desired speed of the motor, (g)means for parameter calculation for generating the informationindicative of both the motor resistance and the motor inductance inresponse to the digitized phase current under predetermined conditions,wherein the means for calculating includes a state machine whichperforms the functions of mechanically stopping rotation of a rotor ofthe motor, applying the drive voltage as a high frequency sinusoidalsignal until the phase current is at a DC value, saving a first value ofthe applied voltage, and saving a resulting value of the digitized phasecurrent, generating the information indicative of the motor resistancefrom the first value of the applied voltage and the resulting value ofthe digitized phase current signal, causing the phase current to beequal to zero, saving a measured value of current change and a secondvalue of the applied drive voltage, and generating the informationindicative of the motor inductance from the measured value of currentchange and the second value of the applied drive voltage.
 17. The systemof claim 16 wherein means for generating a reconstructed digitalrepresentation of a BEMF signal includes a state machine which operatesto capture a digitized phase current, a change in the digitized phasecurrent, and a digitized motor driving voltage, wherein the means forgenerating a reconstructed digital representation of a BEMF signaloperates to compute a value of the BEMF signal in accordance with theexpressionk·v _(bemf) =V·i _(R) ·Δi _(L) −I·v _(R) −V _(L) ·ΔI, where V is thedigitized motor driving voltage, I is the digitized phase current, ΔI isa change in the digitized phase current, i_(R) is a current indicativeof the resistance of the motor, v_(R) is a voltage indicative of theresistance, Δi_(L) is a change of a current indicative of the inductanceof the motor, and v_(L) is a voltage indicative of the inductance of themotor, and k is equal to i_(R)×Δi_(L).
 18. The system of claim 17wherein the state machine operates to cause the synchronization signalto be at a first logic level if a MSB (most significant bit) of the BEMFsignal is a “0” and to cause the synchronization signal to be at asecond logic level if the MSB of the BEMF signal is a “1”.
 19. Thesystem of claim 18 including another state machine for generating theinformation indicative of the resistance and the inductance of the motorin response to the phase current.
 20. The system of claim 16 wherein themeans for generating a signal profile includes an electrical periodmeasuring circuit for measuring an amount of time between correspondingedges of successive pulses of the synchronization signal.